1. Field of the Invention
The present invention relates to an arrival angle estimation system for estimating the angle of arrival of radio waves, a communication device, and a communication system, and particularly to an arrival angle estimation system, a communication device, and a communication system which are used in a wireless communication device incorporating a wireless LAN such as IEEE802.11.
More specifically, the present invention relates to an arrival angle estimation system, a communication device, and a communication system which are used within a short distance range under the condition of sufficient line of sight and are simple, compact, and low cost, in particular, an arrival angle estimation system, a communication device, and a communication system which are used in a wireless communication device equipped with a plurality of antennas.
2. Description of the Related Art
Wireless networks are attracting attention as systems for freeing users from wires in wired communication methods of the related art. Typical examples of such wireless networks are wireless LAN (Local Area Network) specifications such as IEEE (The Institute of Electrical and Electronics Engineers) 802.11a/b, g. Wireless LANs enable flexible internet connections, making it possible to not only replace existing wired LANs but also to provide internet connection means even in public places such as hotels, airport lounges, stations, and cafes.
In many wireless communications, data is transmitted under a multi-path environment in which an overlap of a direct wave and a plurality of reflected waves/delay waves arrives at the receiving device, so delay distortion due to multi-path reception may occur, causing an error in communication. Accordingly, standard wireless LAN specifications such as IEEE802.11a/b, g adopt an OFDM modulation scheme, which is one of multi-carrier schemes. In the OFDM modulation scheme, transmit data is distributed across a plurality of carriers having frequencies orthogonal to each other and then transmitted, so the band of each carrier becomes narrow, thus providing very high efficiency of frequency utilization and robustness against frequency-selective fading.
While wireless LANs have already achieved widespread use, in recent years, it is becoming increasingly common to incorporate wireless LAN functionality also into compact and lightweight CE (Consumer Electronics) apparatuses such as digital cameras, music players, and mobile telephones. Many wireless communication schemes enable flexible connections by use of a non-directional antenna. In contrast, in the case of a wireless LAN incorporated into a mobile apparatus, the size of the antenna must be small. Also, in this case, applications using a directional antenna, such as connecting to a specific communication party who has waved a mobile apparatus over the directional antenna, are conceivable. These applications require a technique for detecting or estimating the angle of arrival of radio waves.
Numerous studies have been done on the techniques for detecting the arrival angle of radio waves, representative examples of which include a method based on the spatial spectrum using the OFDM technique, MUSIC (Multiple Signal Classification) algorithm, and ESPRIT (Estimation of Signal Parameters via Rotational Invariance Technique) algorithm. According to the MUSIC method, by using an array antenna, the eigenvalue and eigenvector of a correlation matrix obtained from an array antenna reception signal are found to calculate an angular spectrum. Although the ESPRIT method also involves finding the eigenvalue and eigenvector of a correlation matrix obtained from an array antenna reception signal as in the MUSIC method, the ESPRIT method differs from the MUSIC method in that the angle of arrival of radio waves is estimated on the basis of the rotational invariance between sub array antennas. Further, an electronically steerable parasitic array radiator antenna (ESPAR antenna) developed by the ATR (Advanced Telecommunications Research Institute International) produces a directional beam with antenna elements arranged in a circle through mutual element coupling, thereby making it possible to steer the angle of arrival of radio waves.
Each of the above-mentioned signal arrival direction estimation techniques provide a high arrival angle estimation accuracy even under multi-path environments, allowing for estimation with an accuracy of ±3 to 5 degrees even in the presence of an obstacle. However, these techniques are designed for measurement between, for example, buildings that are several tens of meters apart. As such, these techniques require an antenna with a special mechanism, such as an array antenna with high dimensionality and large size, and systems using these techniques are expensive due to the complex computations involved, making their incorporation into compact apparatuses difficult.
In the case of a compact apparatus incorporating a wireless LAN, an arrival angle estimation system that is simple, compact, and low cost is desired. The assumed distance between the system serving as the transmitter of radio waves and the system that performs arrival angle estimation is about 2 meters. Since the influence of multi-path need not be taken into account so much in a good line of sight condition, generally, an accuracy of 30 to 40 degrees suffices.
If the distance between two systems is sufficiently short, even when some obstacle is present between the two systems, a human can perform an operation for securing a line of sight by using visual perception to avoid the obstacle. For example, in a case where one of the system serving as the transmitter of radio waves and the system that performs arrival angle estimation is a device that is mobile, if there is an obstacle between the two systems, the user of that system may simply take such actions as moving to a position free from the influence of the obstacle while carrying the mobile device, removing the obstacle, or recognizing that arrival angle estimation does not function and thus not performing an estimating operation.
For example, there has been proposed a signal arrival direction estimation device in which two effective area antennas are each formed as a rigid wave guide horn, the device includes a hybrid circuit for outputting sum and differential signals of a pair of antenna output signals inputted from the two effective area antennas, and the antennas are rotated by 360 degrees about a rotational axis positioned on a line that is perpendicular to a plane including the maximum radiation direction of the two effective area antennas and passes through a point bisecting the electrical centers of the two effective area antennas (see, for example, Japanese Unexamined Patent Application Publication No. 2005-156521). According to the proposed device, the sum and differential signals are measured by separate wideband receivers, the resulting data is integrated in the time domain or the frequency domain to find the total received power for each of angles, and once two patterns with respect to the ultra-wide-band signal are obtained, the direction in which the maximum and minimum values of these two patterns match can be estimated as being the signal arrival direction.
However, it is difficult to incorporate a rotary mechanism that rotationally drives antennas into a compact mobile apparatus, and the presence of such a movable part causes problems in terms of fault tolerance and maintenance. Further, such a signal arrival direction estimation method can be applied only to the receiving side.